1. Technical Field
The present disclosure relates to dilution of engine oil by fuel in a diesel engine.
2. Background Art
Fuel injection strategies in modern diesel engines employ post-injection, i.e., injection during the expansion stroke. Such post-injection is used to increase the temperature of exhaust gases and/or to enrich the exhaust gases to regulate the temperature and stoichiometry in a diesel particulate filter or other exhaust aftertreatment device. Some of the fuel is sprayed on cylinder walls and then is scraped by the oil ring on the piston into the crankcase of the engine. Some of the fuel that is carried into the engine oil in the crankcase evaporates, in particular the lighter ends of the fuel. However, the heavier ends of the fuel remain in the oil and accumulate.
Oil dilution by the fuel can cause the level in the oil pan to exceed the maximum permissible level. This can cause excess foaming of the oil due to the crankshaft dipping into the oil during rotation carrying air into the oil. Oil that has foam has degraded lubricating properties and therefore may shorten the service life of the engine. Furthermore, fuel in the oil reduces the oil's viscosity and degrades the oil's resistance to oxidation. In extreme cases, fuel laden oil is carried into the combustion chamber and affects the power produced by the engine. These problems are exacerbated by the heavier components found in diesel fuel, which do not evaporate as quickly as lighter components.
Computational models of oil dilution are used to estimate the rate at which fuel is added to the oil and the rate at which fuel is removed from the oil by evaporation. Such a model assumes diesel fuel. If, however, a biodiesel fuel is used instead, the model's accuracy suffers. The proportion of heavy components in biodiesel fuel is higher than in conventional diesel fuel. As such, the disadvantages associated with engine oil dilution are exacerbated by the increase in the amount of heavy components as the proportion of biodiesel relative to conventional diesel increases.